1. Field of the Invention
This invention relates to a process for removing chloroprenes from an ethylene dichloride stream containing the same and more particularly to a process for removing the chloroprenes by chlorination to higher boiling chlorocarbons.
2. Description of the prior Art
The process of cracking ethylene dichloride (i.e., 1,2 -dichloroethane) to form vinyl chloride and hydrogen chloride is a well known method of producing vinyl chloride monomer which finds wide use in industry in the preparation of polyvinyl chloride. Generally, only a portion of the ethylene dichloride fed to the cracking furnace is converted to vinyl chloride. The remainder, typically from about 30-70 weight percent of the ethylene dichloride feed, passes through the furnace unreacted and must be recycled for most efficient operation. The ethylene dichloride/vinyl chloride product stream exiting the cracking furnace, which generally contains from about 19 to 44 weight percent vinyl chloride and from 11 to 26 weight percent hydrogen chloride, is typically passed to a series of distillation columns, wherein hydrogen chloride and vinyl chloride are individually recovered in succeeding distillations. The crude ethylene dichloride stream which results as bottoms from the column in which vinyl chloride is collected at the head may then be recovered for recycle to the cracking furnace for production of additional vinyl chloride.
However, recovery of the unreacted ethylene dichloride from the crude ethylene dichloride stream is made more complicated by the presence of impurities which are produced in the cracking step. Of these impurities, the "chloroprenes", are one of the major undesired reaction by-products, and typically comprise up to 1 or more weight percent of the crude ethylene dichloride recycle stream. As used herein, the terms "chloroprenes" and "chloroprene impurities" are intended to include chloroprene (i.e. 2-chloro-1,3-butadiene) and alpha-chloroprene (i.e. 1-chloro-1,3-butadiene). Other impurities which the crude ethylene dichloride stream may contain include such compounds as 1,1-dichloroethane, ethyl chloride and benzene.
The presence of such impurities in an ethylene dichloride stream recycled to the cracking furnace is undesirable since some of these impurities, such as the chloroprenes, have been shown to increase the rate of coke formation in the coils of the cracking furnace, thereby requiring more frequent decoking of these coils, resulting in a loss of vinyl chloride monomer production due to equipment downtime. Thus, it is desired to purify the crude ethylene dichloride stream. Typically, such purification is undertaken by subjecting this ethylene dichloride stream to a series of distillations. In the first distillation, it is attempted to remove those impurities which boil at temperatures lower than that of ethylene dichloride (83.5.degree. C. at 1 atm.) thereby resulting in a distilled bottoms containing the majority of the ethylene dichloride together with high boiling impurities. These bottoms may be passed to a second distillation zone wherein the ethylene dichloride is removed as distillate. While this purification scheme generally results in an ethylene dichloride feed substantially free of the undesired impurities, distillation in the first column, herein termed the "lights column", to remove the low boiling impurities results in a distillate which contains some ethylene dichloride (typically 30 to 60 weight percent ethylene dichloride) in addition to the undesired low boiling impurities such as the chloroprenes. Since in an industrial process discarding this distillate would result in a substantial loss of ethylene dichloride, it is necessary to recycle at least a portion of this distillate for more complete recovery of the ethylene dichloride content thereof. Thus, the bulk of the lights column condensate is typically returned as reflux to the lights column, a small purge stream being generally withdrawn to prevent buildup of the low boilers in the system.
However, such a scheme results in a purge stream from the lights column condensate which may contain up to about 30 weight percent of the chloroprenes, thereby presenting a serious waste disposal problem. Moreover, the chloroprenes present in the condensate can undergo spontaneous polymerization even at room temperature, forming high molecular weight, rubber-like polymers which are insoluble in the organic medium and which tend to foul the lights column and thereby increase the down-time for column cleaning. This spontaneous chloroprenes polymerization is catalyzed by the decomposition of polymeric peroxides which are formed upon reaction of chloroprene with air above temperatures of about 0.degree. C. Thus, it is highly desirable to remove this potential source of column fouling and waste disposal problems.
While processes such as those disclosed in Great Britain Pat. No. 1,323,038 (1973) and U.S. Pat. No. 3,696,015 (P. Wirtz et al. 1972) have been developed for the removal of chloroprenes either prior to or after removal of low boiling impurities from the ethylene dichloride recycle stream, no process has been developed which would avoid the substantial costs due to increased equipment requirements which result from such prior art processes.